Wednesday, November 28, 2007

1.5 Children with bifocals

Before the medical prevention/treatment of myopia becomes a reality and a safe one we hope, we are still stuck with glasses, contacts, or refractive surgery.

Now, if we assume there is a role for accommodation in myopization, then is it possible to use spectacles, instead of cycloplegics, to prevent myopic progression? Yep, it has been done before. And the results apparently were not all that spectacular or you and I would have been wearing bifocals since what, middle school? The problem seems that children with bifocals have a way of peering over the bifocal part to read, plus they lose some navigational agility when, for example, rushing downstairs or playing soccer. The data are highly suspect if the children did not wear those bifocals full time. So a recent clinical study chooses progressive bifocals, i.e., multifocals instead. And the data are far more reliable because of the superb experimental design and diligent follow-ups. The results are a little encouraging or discouraging, depending on your perspective. The bifocaled children in the study do show a smaller increase in myopia comparing with those wearing single-vision spectacles. The difference is, however, less than 0.25D and only during the first year. This therefore does not warrant a change in clinical practice in managing school myopia. On the other hand, "defocus" does have a role in myopia progression. So it is back to chicken or egg first. What in the world triggered this defocussing?

Because of this persisting and may even be correct notion that accommodation is the basis of myopization, over the years, there have been different strategies, e.g., under-correction (common practice in Asia), read without spectacles, etc. Mostly for parents' psychological benefits, though.

The math of accommodation is quite simple. For reading at 13 inches (i.e., 40 cm or 0.4 m), you'll need to focus 1/0.4 = 2.50D and so on. You start out as a child with a full accommodative power of around 14-16D which decreases with age pretty much linearly until about 0.25D when you are 70 years old. It depends on how much accommodative reserve your have left. Let's say 1.50D, then you will need either longer arms to hold your newspaper at (1/1.5=) 67 cm, or another 1D in the form of reading glasses to make up the difference - in order to read at 40 cm. By the same token, if 2.50D is to be eliminated from children's near focusing, then 2.50D is provided for in those bifocal/multifocal clinical trials. And if 2.50D is needed for reading at 40 cm, why read at 20 cm as some children do, with (1/0.2 =) 5D, or 5 - 2.50 = 2.50D extra, unnecessary accommodation for reading the same Harry Potter thing.

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